My race car has 0.0029"-0.0035" main clearances and 0.0024-0.0026" rod clearances. I use Walmart 10W-40 full syn and it has 30 psi at full heat soaked 1000 rpm idle. I have had it apart and all the bearings looked good. Oil flow is just as important as viscosity and viscosity requirements are more dependent on bearing loads. For my power level (~700 Hp) what I use is fine. I use 5W-30 full syn in my street cars.
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1967 Firechicken, 499", Edl heads, 262/266@0.050" duration and 0.627"/0.643 lift SR cam, 3.90 gear, 28" tire, 3550#. 10.01@134.3 mph with a 1.45 60'. Still WAY under the rollbar rule.
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